Vehicle air conditioning systems include an evaporator from which refrigerant in a vapor-liquid mixture, including entrained lubricant, flows as the system operates. It is pulled from the evaporator under substantial suction by the compressor. With compressors of the fixed displacement type, that suction pressure remains high the entire time that the compressor is engaged. Because the compressor does not operate well if liquid refrigerant is drawn directly into it, an accumulator is interposed between the evaporator and compressor. The accumulator is typically a hollow cylinder, vertically mounted beneath the vehicle hood, with an inlet from the evaporator opening directly into the canister at a high point. The outlet to the compressor from the canister is indirect, through a return tube internal to the canister. The return tube has one end located just below the top of the canister, and the other end exits the canister and is connected directly to the compressor suction. Refrigerant-lubricant mixture entering the canister fills the canister from the bottom up, leaving a vapor space at the top, surrounding the open end of the return tube. Compressor suction is therefore applied to the vapor space, and vaporous refrigerant is drawn out. The accumulated liquid refrigerant and lubricant is drawn into the return tube through a small diameter metering orifice located near the bottom end of the canister, and effectively atomized, so that dense liquid does not enter the compressor.
The shape of the return tube within the canister is dictated by underhood space and compressor line routing. Ideally, the return tube will be able to exit the canister from a high point, comparable to the inlet. If so, the return tube can be given a U shape, with the bend or bight of the U located near the bottom of the canister, incorporating the bleed orifice. This gives the advantage of acting like a sink trap, and preventing drain down through the orifice into the compressor when the compressor is not operating. In some circumstances, the only feasible exit point for the return tube is out the bottom of the canister, usually the bottom center. In that case, the return tube is straight, not U shaped, and there is no sink trap effect. The bleed orifice still must be near the bottom of the return tube, so it is possible for liquid to drain down through the orifice, small as it is, given enough time. One possible solution to this problem is disclosed in co-assigned U.S. patent application Ser. No. 08/148,236, U.S. Pat. No. 5,347,829. There, a novel return tube has a closely packed bent shape which places a sink trap type bend inside the canister, even though the exit to the compressor is through the bottom of the canister. Therefore, drain down is prevented. However, that particular return tube may not fit in all bottom feed canisters.
Another possible solution to drain down in a bottom feed accumulator is disclosed in co pending, co assigned U.S. application Ser. No. 08/130,399, filed Oct. 1, 1993. Instead of a bleed orifice near the lower end of the straight return tube, a separate, small diameter pick up tube runs parallel to the return tube from a pick up point at the bottom of the canister and into the return tube at a high point. Compressor suction acts through the pick up tube to draw out accumulated liquid. When the system is off, liquid can accumulate to a height equal to the point where the pick up tube enters the return tube, without draining into the return tube. Not all compressors may provide enough suction to act through the entire length of the thin pick up tube, however.